This invention relates to a method of reducing "skew" in data carrying cables, and to cables and data installation in which the method of the present invention is used to reduce skew to an acceptable level.
For the purposes of data transmission, cables comprising a plurality of pairs of conductors are frequently used. Each conductor is separately insulated and each pair of conductors is twisted together to form a so-called "twisted pair". The "twist-rate" (that is the number of twists per unit length) of each pair is deliberately different from that of each other pair so as to reduce cross-talk between the pairs. The effect of this variation in twist rate is that for any given length of cable the length of the conductors forming each twisted pair will be different from the length of conductors forming each other twisted pair. As a result of this length variation, signals applied simultaneously to the twisted pairs at one end of the cable will arrive at different times at the other end of the cable. This characteristic is known as "skew".
By way of example, the transit time for signals injected simultaneously on two different pairs of a typical commercial Category 5 (100 MHz) cable containing four twisted pairs and a length of 100 m may typically vary by 20 ns (20 nano seconds). Such variation in transit time renders cables of this length unacceptable for high frequency data transmission, and in particular, such cables are totally unacceptable for so-called "gigabit ethernet" systems.
With a view to avoiding this problem, special cables have been produced for high speed data transmission. Such cables are of complex construction and typically include individually shielded conductors which are bonded together in a manner in eliminate length variations of the conductors within a given length of cable. Such cables are expensive and difficult to use because of their limited flexibility and because of the relative complexity of forming terminations on the cables.